Magnetoresistive sensor scanning a magnetic multipole wheel

ABSTRACT

According to the invention, a magnetoresistive sensor for scanning a magnetic multipole wheel is proposed, which scans many uniformly disposed magnetic pole pairs. The sensor has four magnetoresistive resistors, which are connected electrically as a Wheatstone bridge with two half-bridges and which output a sine-wave and cosine-wave signal, respectively. To suppress the third and/or fifth harmonic of the scanning signal, it is proposed that the spacing between the two half-bridges be selected such that it is not equal to half the wavelength (lambda). For the third harmonic, the spacing d=lambda/3 is preferably selected, so that the half-bridges are offset by a phase angle of 120°. To suppress the fifth semiharmonic, three identical sensors with a suitable spacing are connected together. The measured signals are averaged, so that the result is a field course with the third and fifth harmonics filtered out.

PRIOR ART

[0001] Position and angle measurement by scanning magnetic scales or multipole wheels is described in the prior art, for instance in German Patent Disclosure DE 195 06 938 A1.

OBJECT OF THE INVENTION

[0002] At the distances, or spacings, that result in practice between a magnetic sensor and a scale, the course of the magnetic field is not ideally sinusoidal—as assumed—but instead includes square-wave components, which can be generally well described by a third and fifth harmonic of the sine-wave function. This limits the precision of the method, however. A field course free of harmonics generally exists only at relatively great operative spacings and thus at very low working field intensities (the field intensity drops off exponentially with the distance), which makes the system very vulnerable to malfunction.

[0003] The invention proposes a sensor structure which on the basis of its geometry filters the third and/or the fifth harmonic out of the signal, so that a sinusoidal course results as the measured value, even for only slight spacings between the sensor and the scale. As a result of the aforementioned harmonic correction, the course is moreover largely independent of the working spacing of the sensor, so that even the influence of an eccentricity that may be present in the system on the outcome of measurement is minimized.

SUMMARY AND ADVANTAGES OF THE INVENTION

[0004] Position transducers with improved precision and vulnerability to malfunction by operation at small working spacings. Insensitive to eccentricities of the pole wheel or to its position.

DETAILED DESCRIPTION OF STRUCTURE AND FUNCTION

[0005] It is state of the art to scan magnetic scales (such as multipole wheels) using so-called gradiometers; see also FIG. 1. This involves a Wheatstone bridge circuit, comprising four identical magnetoresistive elements, with the two half-bridges spatially separated; see FIG. 2. Each individual element of the bridge furnishes a voltage as a function of the magnetic field applied to it. We assume that the output voltage of each individual element of the bridge varies sinusoidally with the directional angle of the magnetic field applied. This is particularly true if a so-called spin valve is used as the magnetoresistive element. If the scale in this case moves past the sensor by a spacing of one pole pair, this is equivalent to a phase of 360° of the signal at the single resistor. The spacing of the two half-bridges, symbolized by the letter d, is typically half the length of one pole pair, that is, lambda/2 (see FIG. 1), since the phases of the signals at the half-bridges are phase-offset by 180°, and thus a sinusoidal bridge output signal of maximum amplitude is available.

[0006] The invention now makes use of the fact that

[0007] 1. even at a spacing d of the two half-bridges not equal to lambda/2, in most cases a sinusoidal bridge signal

[0008] 2. is available, although with a slightly reduced amplitude;

[0009] 3. the bridge for harmonics of period d furnish no signal.

[0010] For reasons of symmetry, with the aforementioned pole wheels, only odd-numbered harmonics occur. If one seeks to make a bridge that suppresses the third harmonic, then the spacing d=lambda/3 (120°), or a multiple thereof, should be selected. In that case, the phase between the half-bridges is 120°, which is precisely equivalent to the period of the third harmonic. In that case, the bridge voltage is reduced to 90%, compared to the bridge with d=lambda/2, and the bridge signal is offset by a phase of 30° compared to the fundamental of the original signal; see FIG. 3.

[0011] The fifth harmonic can be suppressed in the same way by means of a spacing of the half-bridges of d=lambda/5, or a multiple thereof; see FIG. 4.

[0012] In practice, sufficiently good harmonic suppression is achieved if the third and the fifth harmonics are suppressed. This can be achieved by the following arrangement and signal evaluation (see FIGS. 5 and 6):

[0013] 1. The signal S₁ of a bridge B₁ with d=lambda/3 is averaged with the signal S₂ of a second bridge B₂ with d=2*lambda/3; the resultant signal is S₁₂.

[0014] 2. A signal S₃ of a bridge B₃ with d=lambda/3 is also available, which is phase-offset from B₁ and B₂ by 30°. This is accordingly the phase-corrected signal of the 120° bridge.

[0015] 3. The mean value between S₁₂ and S₃ results in the field course, with the third and fifth harmonics filtered out. 

1. A magnetoresistive sensor for scanning a magnetic multipole wheel (1), which has many uniformly disposed pole pairs (2), having four magnetoresistive resistors (R1 . . . R4), which are connected electrically as a Wheatstone bridge with two half-bridges (R1, R3; R2, R4, respectively) and output a sine-wave and cosine-wave signal, respectively, characterized in that the spacing (d) between the two half-bridges (R1, R3; R2, R4, respectively) relative to the scanning direction of the multipole wheel (1) is selected such that it is not equal to half the wavelength (lambda) of one pole pair (2).
 2. The sensor of claim 1, characterized in that the spacing (d) is selectable such that it is equivalent to one-third of the wavelength (lambda).
 3. The sensor of claim 1, characterized in that the spacing (d) is selectable such that it is equivalent to one-fifth of the wavelength (lambda).
 4. The sensor of claim 2 or 3, characterized in that the spacing (d) is equivalent to a multiple of the fractional wavelength.
 5. The sensor of one of the foregoing claims, characterized in that a second sensor (3) can be provided; that the second sensor (3) is likewise electrically embodied as a Wheatstone bridge with two half-bridges; and that the spacing of the half-bridges of the first sensor is d=lambda/3, and the spacing of the half-bridges of the second sensor is d=2*lambda/3.
 6. The sensor of claim 5, characterized in that the signals of the two bridges are averaged.
 7. The sensor of claim 5 or 6, characterized in that a third sensor with an identically constructed bridge is provided downstream, in which the spacing is d=lambda/3; and that the third sensor is phase-offset by 30° from the first and second bridges.
 8. The sensor of one of the foregoing claims, characterized in that the average value can be determined from the signals of the three bridges.
 9. The sensor of one of the foregoing claims, characterized in that the sensor can be used to determine an angle of rotation. 